how to add hidden in upload image form

hi, all

I want to use <input type="hidden"> in upload image form,
for tracking something,

how shall I to do this? how to add it.

i try this:

var infoTab = dialogDefinition.getContents( 'Upload' );
      infoTab.add( {
         id : 'request_token',
         type : 'text',
         name: 'test',
         label : 'label',
         'default':'123'
      },"uploadButton");

but this not insert into the upload form.

thanks.